1. Field of the Invention
The present invention relates to an LED (light-emitting diode) driver and, more specifically, to an LED driver with multiple feedback loops.
2. Description of the Related Arts
LEDs are being adopted in a wide variety of electronics applications, for example, architectural lighting, automotive head and tail lights, backlights for liquid crystal display devices, flashlights, etc. Compared to conventional lighting sources such as incandescent lamps and fluorescent lamps, LEDs have significant advantages, including high efficiency, good directionality, color stability, high reliability, long life time, small size, and environmental safety.
LEDs are current-driven devices, and thus regulating the current through the LEDs is an important control technique for LED applications. To drive a large array of LEDs from a direct current (DC) voltage source, DC-DC switching power converters such as a Boost power converter is often used with feedback loops to regulate the LED current. FIG. 1 illustrates a conventional LED driver using a Boost converter. The LED driver includes a Boost DC-DC power converter 100, coupled between input DC voltage Vin and a string of LEDs 110 connected to each other in series, and a controller circuit 102. As is conventional, the boost converter 100 includes an inductor L, diode D, capacitor C, and a switch S1. The boost converter 100 may include other components, which are omitted herein for simplicity of illustration. The structure and operation of the boost converter 100 is well known—in general, its output voltage Vout is determined according to the duty cycle of the turn-on/turn-off times of switch S1. The output voltage Vout is applied to the string of LEDs 110 to provide current through the LEDs 110. The controller circuit 102 detects 104 current through the LEDs 110 and generates a control signal 106 based on the detected current 104 to control the duty cycle of the switch. The controller circuit 102 may control the switch S1 by one of a variety of control schemes, including pulse width modulation (PWM), pulse frequency modulation (PFM), constant on-time or off-time control, hysteretic/sliding-mode control, etc. The controller circuit 102 and the signal paths 104, 106 together form a single feedback loop for the conventional LED driver of FIG. 1. The two main challenges to conventional LED drivers, such as that shown in FIG. 1, are speed and current sharing.
Fast switching speed is required in the LED driver, because the LED brightness needs to be adjusted at a frequent rate. Fast switching speed is particularly useful for dimming control with pulse-width modulation (PWM), where the LED needs to transition from light or no load to heavy load and vice versa in short time. The speed of an LED driver is a measure of its small-signal performance. Because of the inherent right-half-plane (RHP) zero in the Boost converter, the speed of conventional LED drivers is limited below what most LED applications require.
Current sharing is needed because of parameter variability of LEDs caused by their manufacturing processes. When multiple series-strings of LEDs are connected in parallel, a small mismatch in the forward voltage (VF) of the LEDs can cause large difference in their current brightness. Current sharing has been attempted in a variety of ways. One rudimentary approach is to drive each of the multiple LED strings with a separate power converter. However, the disadvantage of such approach is obviously high component count, high implementation cost, and large size.
Another approach is to use current mirrors each driving one LED string, for example, as shown in U.S. Pat. No. 6,538,394 issued to Volk et al. on Mar. 25, 2003. However, a disadvantage of such current mirror approach is that it has low efficiency. That is, when the forward voltages of the LEDs differ, the output voltage (V+) of the power converter applied to the parallel-connected LED strings has to be higher than the LED string with the highest combined forward voltage ΣVF. There is a voltage difference (V+−ΣVF) in the LED strings with a combined forward voltage lower than the highest, which is applied across each current mirror, with the highest voltage difference being present in the LED string with the lowest combined forward voltage ΣVF. Since the power dissipated by the current mirrors does not contribute to lighting, the overall efficiency is low, especially when the difference in the combined forward voltage between the LED strings is large.
Still another approach is to turn on each of the multiple LED strings sequentially, as shown in U.S. Pat. No. 6,618,031 issued to Bohn, et al. on Sep. 9, 2003. However, this approach requires even faster dynamic response from the LED driver, and thus forces the power converter to operate in deep discontinuous mode (DCM), under which power conversion efficiency is low.